Ion exchange polymer membranes have found utility in a number of electrochemical and other processes. One use has been as membranes for solid polymer electrolyte cells. Solid polymer electrolyte cells typically employ a membrane of an ion exchange polymer which serves as a physical separator between the anode and cathode while also serving as an electrolyte. These cells can be operated as electrolytic cells for the production of chemical products or they may be operated as fuel cells for the production of electrical energy. Ion exchange polymer membranes are also used for facilitated transport, diffusion dialysis, electrodialysis, pervaporation and vapor permeation separations.
Polymer ion exchange membranes must have sufficient strength to be useful in these various applications. Membranes of highly fluorinated polymers such as perfluorinated sulfonic acid polymer membranes are particularly well-suited for use in such cells due to excellent chemical resistance, long life, and high conductivity. However, for some applications, the tensile strength of such membranes is not as high as desired. If increased physical strength is achieved by making the membrane thicker, a decrease in ionic conductance may result. Reinforcement are sometimes incorporated into the membranes to increase strength. For example, in membranes used in the chloralkali process, i.e., the production of caustic and chlorine by electrolytic conversion of an aqueous solution of an alkali metal chloride, woven reinforcements are incorporated into the membranes. For other applications such as in fuel cells, increased tensile strength is typically not needed in use but may be desirable for ease of handling or for certain manufacturing operations involving the membranes.
Reinforcement of polymer ion exchange membranes to confer improved physical properties has been disclosed in a number of prior art references. A number of references disclose combining porous expanded polytetrafluoroethylene (EPTFE) with highly fluorinated ion exchange polymer. For example, in U.S. Pat. No. 3,692,569 (Grot), surface active fluorocarbon objects are described as a composite structure of an inert fluorocarbon polymer core with a chemically modified fluorocarbon copolymer surface which confers chemical activity to the structure. Exemplified is a fluorocarbon core which is a porous support film of polytetrafluoroethylene and which is coated with a fluorocarbon copolymer with pendant chemically active sulfonyl groups. The coating copolymer either forms a coating in the pores or fills the pores depending on the desired use of the structure.
U.S. Pat. No. 4,902,308 (Mallouk et al.) describes a sheet of porous expanded polytetrafluoroethylene coated with a reactive metal salt of perfluorinated ion exchange polymer for use in scavenging unwanted gas components.
U.S. Pat. No. 5,082,472 (Mallouk et al.) discloses a dimensionally stable composite membrane for use in facilitated transport unit operations. The composite is described as a porous expanded polytetrafluoroethylene in laminar contact with a continuous perfluorinated ion exchange resin layer, the ion exchange layer being swollen with a hydrophilic liquid.
U.S. Pat. No. 4,954,388 (Mallouk et al.) discloses an abrasion-resistant, tear resistant, multilayer composite membrane for use in electrolysis. The composite membrane is described as having a perfluorinated ion exchange layer attached to a reinforcing fabric by means of an interlayer of porous expanded polytetrafluoroethylene.
U.S. Pat. No. 5,547,551 (Bahar et al.) discloses an ultra-thin composite membrane of porous expanded polytetrafluoroethylene impregnated with ion exchange polymer. The base material is defined by a thickness of less than 1 mil (0.025 mm) with the ion exchange resin impregnating the membrane such that the membrane is essentially air impermeable.
The process for producing porous expanded polytetrafluoroethylene support films used in reinforcing ion exchange membrane is described in U.S. Pat. Nos. 3,953,566 and 3,962,153 (both to Gore). The references teach a method of rapidly stretching a paste-formed film of highly crystalline PTFE and then heat treating (or sintering) it to increase the amorphous content of the PTFE to typically greater than 10%. The resultant structure is expanded, amorphous-locked polytetrafluoroethylene film having a porous microstructure of polymeric fibrils which is easily bonded to other materials.
Although the prior art has shown ways of strengthening ion exchange membranes by reinforcement porous expanded PTFE supports, tear strength has not been as high as is desirable. A composite membrane is needed for application in electrochemical processes which possesses greater tear strength without sacrificing performance.